Artificial seaweed

ABSTRACT

AN IMPROVED FORM OF &#34;ARTIFICIAL SEAWEED&#34; FOR COMBATING COASTAL EROSION AND THE LIKE COMPRISES AN ANCHORED ARRAY OF FILAMENTARY STRANDS OF FOAMED, STRETCHED POLYOLEFIN, CHARACTERIZED BY INTERNAL PLEXIFORM STRUCTURE SURROUNDED BY A SUBSTANTIALLY CLOSED, THIN SKIN, HAVING A TENSILE STRENGTH OF AT LEAST ABOUT 1 GRAM PER DENIER AND A FINAL DENSITY BELOW 500 G./1. ING COASTAL EROSION AND THE LIKE COMPRISES AN ANCHORED ARRAY OF FILAMENTARY STRANDS OF FOAMED, STRETCHED POLYOTEFIN, CHARACTERIZED BY A INTERNAL PLEXIFORM STRUCTURE SURROUNDED BY A SUBSTANTIALLY CLOSED, THIN SKIN, HAVING A TENSILE STRENGHT OF A LEAST ABOUT 1 GRAM PER DENIER AND A FINAL DENSITY BELOW 500 G./1.

United States Patent 01 :"fice 3,559,407 ARTIFICIAL SEAWEED GerritSchuur, Delft, Netherlands, assignor to Shell Oil Company, New York,N.Y., a coi'poration of Delaware No Drawing. Filed Nov. 25, 1968, Ser.No. 778,757

Int. Cl. E02]: 3/00 US. Cl. 61-3 f 3 Claims ABSTRACT OF THE DISCLOSUREThis invention is directed to an improvement in artificial seaweed,comprised of assemblages of polyolefin strands. Such assemblages areuseful as a means for influencing the migration of material at thebottom of bodies of water, as in combating coastal erosion.

As known heretofore and described, for example, in British Pat. 984,077to Roblon, such a protective assemblage of artificial seaweed consistsof a wide screen formed by a large series of filamentary plasticelements or strands which are secured at one end to an anchoring meansto be placed at the bottom of the sea. The strands have a lower densitythan water so that the screen formed of these elements will assume andretain an upright position in the Water, thereby reducing currents inthe surrounding water and promoting the deposition of sand or othersolid materials entrained by the water. Erosion of sea-floors in coastalwaters, which is sometime a serious problem in the absence of seavegetation, can thus be successfully combated. A similar problem isexperienced with structures erected on the sea-bottom, likelandingstages, piers and fixed drilling platforms, where strong currentsand turbulence around the foundation piles and beams may erode thesea-bottom and wash out the foundation. By providing a zone protected byartificial seaweed around the bottom end of the support members of thefoundation, the scouring action of the sea no longer endangers thestability of the foundation.

The filamentary elements known heretofore can be formed by a pluralityof single or composite strands of a thermoplastic material. The strandsmay be solid material if the density of this material is less than thatof water. It has also been proposed to use single hollow fibers closedat either end.

None of the heretofore disclosed materials has proved very satisfactoryfor use as artificial seaweed. Some of the elements lack strength orsuflicient buoyancy, others do not have the desired low density or, asin the case of the individual hollow fibers, are expensive to make.Since any adequate protection of a coastal area requires huge quantitiesin volume of the seaweed, only low-cost, easyto-prodnce materials arefeasible for this purpose.

It is the object of the invention to provide artificial seaweed thatperforms well for prolonged periods and which can be made simply and atlow cost.

Artificial seaweed, according to the invention, comprises a plurality ofelongated, flexible, buoyant strands, such as filaments or tapes,secured to anchoring means adapted to maintained the elements near thebottom of a body of water, said elements having been formed by extrudingmulticellular foam strands of a polyolefinic material having a densityless than 300 g./l., and subsequently stretching the strands in a ratioof at least 5:1. The result- 3,559,407 Patented Feb. 2, 1971 ing strandsare thin elongated flexible elements having an open plexiform networkstructure surrounded with a substantially closed, thin-walled skin.

The material of the elements is preferably polypropylene. The extrudedpolypropylene foam strands have a cellular structure. The extrusiveconditions are controlled to provide strands having a measured densityof no more than 300 g./l. The extruded, unstretched strands arerelatively weak, having typically a tensile strength of about 0.1g./denier. After being stretched at a ratio of at least 5:1, the strandshave an open, plexiiorm structure rather than a cellular structure, andhave a greatly improved tensile strength, for example, between 1 and 5grams per denier. Therefore, the stretched foam strands are tens oftimes stronger than the unstretched foam strands. The improved tensilestrength is not only of importance to prolong the useful life of theseaweed under water but also to avoid excessive waste by breakage duringmanufacture of the seaweed, when the elements are being tied to ananchoring device, and during the subsequent handling and transportation.

In spite of the open internal structure of the elements, no substantialvolume of water can penetrate into them because they have a relativelyunbroken outer skin and because polypropylene is a nonwater absorbentmaterial. The air within the elements remains entrapped even underrelatively highfluid pressures and continues to contribute greatly tothe buoyancy of the elements.

Foamed strands suitable for conversion to the articles of this inventionare produced by the methods described in detail in Netherlands patentapplications 6511455, published Mar. 3, 1967 and 6610834, published Feb.5, 1968. In these methods, polypropylene is admixed with a volatilizablefluid blowing agent which expands when a melt of polypropylene andblowing agent is extruded into a zone of lower pressure, e.g., from anextruder into the atmosphere. A small proportion of a chemicallydecomposable blowing agent may be present as a foam nucleating agent forthe volatilizable fluid blowing agent.

The stretching is suitably performed at elevated temperatures, forpolypropylene usually between and C. The stretching ratio of theextruded foam strands may be between 5:1 and 15:1, and preferablybetween 7:1 and 10:1.

The length of the elements can be selected to requirement, but anaverage will be from 3 to 10 feet. Since the elements have a lowerdensity than water, they must be anchored to the sea floor. Variousanchoring means may be selected; for example, the elements may besecured at one end in bundles to concrete blocks. It is desirable thatthe elements form an uninterrupted screen in the water, and therefore,the elements are preferably connected in closely related lineararrangement to an elongated anchoring device, such as a steel cable.Each element can be clamped to the cable at a middle portion thusleaving both ends free and seemingly forming two elements of half theoriginal length. Other suitable anchoring devices are chains, rings, andsmall sand bags. The elements can be arranged individually, or a unitarystructure of woven or otherwise interconnected elements can be formed.

In practice, stretched and extruded polypropylene foam strands,according to this invention, were found to perform very well as anartificial seaweed; the problem of vulnerability and easy breakageexperienced with unstretched foam strands was overcome by the stretchingoperation. Although the density of the foam strands increases when beingstretched, this imposes no problem at all since the density of thestretched elements still remains below 500 g./l., and hence much lessthan that of water. For example, isotactic polypropylene (melt index 8)foam strands having a density after extrusion of 34 g./l. were stretchedat a ratio of 13:1 at 164 C. and thereby obtained a final density of 178g./l. In another case, polypropylene foam strands with a density of 28g./l. were stretched at a ratio of 9:1 at 162 C., resulting in a finaldensity of 107 g./l.

The final density of a stretched foam strand was calculated bydetermining its weight and volume, the latter established by submersionin a water bath. Since the strands have their open net-structuresurrounded with a predominantly closed skin, little water will penetrateinto the strands. However, since the volume measurement was made using arelatively short piece of strand, the water that might enter the strandat the ends thereof, where no protective skin is present, mightinfluence the volume measurement too much, and for this reason, the endsof the sample strand had been scaled.

The foamed strands are suitably in the form of tapes having incross-section a greatest dimension of 2.5 mm. and a smallest dimensionof 0.5 mm. Generally satisfactory dimensions for the strands are a widthbetween 1 and 10 mm. and a thickness between 0.25 and 4 mm.

I claim as my invention:

1. Means for influencing the migration of material at the bottom ofbodies of water, comprising an array of elongated, flexible, polyolefinstrands having a tensile strength of at least 1 g./ den. and an internalplexiform structure surrounded by a substantially closed thin skin, saidstrands having been formed from extruded multicellular foam strands of apolyolefinic material, having a density less than 300 g./l., bystretching in a ratio of at 5:1, and being secured to anchoring meansadapted to maintain the array near the bottom of a body of water.

2. Article according to claim 1 wherein said polyolefin ispolypropylene.

3. Article according to claim 2 wherein said strands are tapes having,in cross-section, a greatest dimension in the range from 1 and 10 mm.and a smallest dimension in the range of 0.25 to 4 mm. and a densityless than 500 grams per liter.

References Cited Man-made Fibres by R. W. Moncrieff, published in theUSA by John Wiley & Sons, Inc., New York.

PETER M. CAUN, Primary Examiner

